The present invention is directed to an improved ultraviolet (UV) light emitting diode. More specifically, the present invention is related to an improved UV light emitting diode which can be manufactured without a low temperature buffer layer thereby providing improved efficiency, improved processing and an enhanced ability to incorporate heat dissipation technology.
UV light emitting diodes are highly desirable for a number of applications and proposed applications. They are expected to find great utility in such diverse areas as bio-chemical sensors, air and water purification, food processing and packaging, and various forms of medical applications such as dentistry, dermatology and optometry.
The shorter wavelength radiation is also expected to be advantageous for use in optical data storage systems such as optical disk wherein the data density can be increased over longer wavelength systems. Other areas could also benefit from a high energy, efficient, UV diode.
UV and deep-UV diodes typically utilize unitary, binary, ternary, quaternary and penternary group III metal nitrides such as AlN, GaN, InN, AlGaN, AlInN, GaInN, AlGaInN and BAlInGaN. The inherent properties of these materials have thwarted full exploitation of UV emitting diodes. These materials have inherently high electrical resistance and low thermal conductivity. As a result the amount of current which can be supplied to the diode is limited by internal heating which decreases the amount of light that can be generated relative to visible emitting diodes.
Yet another problem with group III nitrides is the lack of a suitable support upon which the materials can be grown. The most common supports, such as sapphire, have lattice parameters which are sufficiently different from the group III nitrides that crystal defects are generated at the interface with the substrate. The defects propagate into the active layers of the diode structure thereby further decreasing efficiency. Also, due to the difference in the coefficient of thermal expansion of group III nitrides, relative to typical supports, significant crack formation occurs during subsequent processing.
Lattice defects and thermal expansion cracking are typically mitigated by a low temperature buffer layer which is grown at a relatively low temperature of 400 to less than 700° C. between the substrate and the active layers of the diode. Low temperature buffer layers are typically a relatively soft material, such as AlN, wherein the thermal expansion differences are dampened. Buffer layers also have the advantage of significantly reducing lattice defects. The lattice defects are effectively terminated within the buffer layer and therefore do not reach the active layers.
The diode is often separated from the substrate by cleavage at the low temperature buffer by known techniques such as laser lift-off. Unfortunately, the cleavage is not clean and typically some fraction of the low temperature buffer layer material remains on the substrate with an uneven surface remaining on the diode. The uneven exterior of the diode further decreases diode quality and efficiency due to light attenuation and light scatter.
A large portion of the research efforts related to UV diodes has focused on improving the buffer layer, methods for forming the buffer layer and manufacturing a ultraviolet light emitting diode. Pulsed growth techniques have been demonstrated to be particularly beneficial. A particularly efficient buffer layer is formed by a pulsed lateral overgrowth (PLOG) technique described in commonly assigned U.S. patent application Ser. No. 12/445,959, now U.S. Pat. No. 8,304,756, and which corresponds to WO 2008/054994, both of which are incorporated herein by reference. Another method for forming a buffer layer is pulsed atomic layer epitaxy (PALE) which is described in commonly owned application. WO2009/023722, which corresponds to U.S. Pat. No. 8,354,663, which are both incorporated herein by reference. Another method of improving the stability of the device, is forming an array of light-emitting devices (LED) and a method for making an array of LED's, particularly one that emits deep ultraviolet light which is described in commonly owned application WO2009/023722, cited above.
Even with the highly advanced methods of forming buffers the problems associated with thermally induced cracks and crystalline defects from lattice mismatches remains. These problems, though mitigated, still limit the full exploitation of UV diodes.
There has been an ongoing effort to further mitigate the deficiencies described above. The present invention advances the art by techniques which are considered to be a contrary approach.
However, there remains a need for a higher quality, more reliable, more robust, deep UV light-emitting diodes and laser diode arrays.
Milli-watt power DUV LEDs on sapphire substrates with AlGaN multiple quantum well (MQW) active regions have been previously reported for the UVA, UVB and the UVC regions. The LED design used in the prior art benefited from several key innovations, namely: (1) the use of pulsed atomic layer epitaxy (PALE) to improve the quality of the buffer AlN layer; (2) the use of a PALE deposited AlN/AlxGa1-xN, short-period super-lattice layer insertion between the buffer AlN and the n-contact AlGaN layer for controlling the thin-film stress and mitigating epilayer cracking; and (3) a p-GaN/p-AlGaN hetero-junction contact layer for improved hole injection.
To date, under a cw-pump current of 20 mA, the average output powers for state-of-the-art UVC and UVB LEDs are about 1 mW. These LEDs typically have effective areas ranging from approximately 200 μm×200 μm to 300 μm×300 μm with various geometrical shapes demonstrated. Due to the poor thermal conductivity of the sapphire substrates, the output power quickly saturates at pump currents around 40-50 mA. At 20 mA pump current, the device lifetimes (50% power reduction) are approximately 1000 h for packaged devices that are flip-chipped to a heat sink. Without being constrained by theory, the key reasons for this power/lifetime limitation are the dislocations in the active region and the excessive heating due to the high device series and poor thermal conductivity of sapphire. Unfortunately, many commercial applications require the output powers and lifetimes to be significantly better than the best values reported to date.
The high operating voltages, or high series resistance, of deep UV LED's stems from poor or lower doping efficiency of high aluminum content epilayers needed for such short wavelength emission. The series resistance further increases the temperature rise of the active junction by joule heating which then results in degraded device performance. The problem becomes severe with decreasing emission wavelength and with increasing device area. Increasing the device active area has adverse effects due to the severe crowding in high aluminum content layers. The present invention solves this problem.
The present invention reduces series resistance which results in a decrease in joule heating. In the micro-pillar design the diameter is within the charge spreading length. For example, for a 280 nm UV LED the mesa diameter is preferably about 25-30 μm which is less than, the estimated current spreading length of 40 μm. Furthermore, the micropillar is surrounded by n-type electrode with a small distance of separation between the pillar edge and the n-type electrode edge. Since these micropillars are immersed in a pool of n-type electrode and the diameter is less than the current spreading length the current crowding problem is eliminated. Moreover, by interconnecting each pillar with the novel flat electrode over a leakage suppression layer the total device area can be increased which reduces the device resistance and operating voltages. As a consequence of this reduction, the device is much cooler than conventional UV LED's which helps in biasing these UV LEDs to much higher drive currents for a stable high power performance.
It will be apparent to those skilled in the art of ultraviolet light-emitting diodes and laser diodes that many modifications and substitutions can be made to the preferred embodiments described herein without departing from the spirit and scope of the present invention, defined by the appended claims.